Tuan,

We noticed the same thing a few years ago.

The impact soldermask has on impedance is dependent on the ratio of
soldermask thickness to FR4 thickness (or other PWB material).  If the
thickness FR4 is thin, the addition of soldermask has a larger impact on the
dielectric constant than if the FR4 is thicker.  You are looking for the
change in the dielectric constant by adding soldermask, not necessarily what
the exact value is.  If the FR4 is 0.002" thick, soldermask will have a
drastic reduction in the impedance as compared to 0.008" thick FR4.

It will also depend on the conductor width.  As the conductor gets small, a
higher percentage of the capacitive field go out the edge of the conductor,
through the soldermask, before returning to the plane.  When the conductor
is wide, the fringe field is still there.  However there is a much larger
portion of the capacitance that is directly coupling from the conductor,
through the FR4, to the plane.  This reduces the impact of the soldermask.

Look at the details of the microstrip structure.  If your target is 50 ohms
and the trace width is less than 5 mils, it wouldn't surprise me to see the
reduction you mentioned by adding soldermask.

FYI,

Darren

        ----------
        From:  Tuan Le [SMTP:[log in to unmask]]
        Sent:  Thursday, February 17, 2000 11:13 AM
        To:  [log in to unmask]
        Subject:  [TN] Soldermask effect on impedance

        Hi everyone, 

        I have done some tests recently by measuring impedance (on traces
from the coupons) before and after soldermask.  I found that there are quite
a large drop in impedance after soldermask: 4 to 6 ohms for single ended and
8 to 10 ohms for differential impedance.  Has anyone experienced this? 
Could anyone offer me an explanation of what happened?  We use the same
Taiyo product for a long while.

        Your help is greatly appreciated. 

        Tuan Le 
        [log in to unmask] 
	

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